The present invention generally relates to rotational load absorption devices for measuring the power output of a prime mover and more specifically is directed to an improved Prony brake-type load absorption device or dynamometer providing a longer operating life, increased load measuring capacity and safer, more economical operation.
A Prony brake generally includes a rotating brake drum or disc connected with the output shaft of a prime mover, such as an internal combustion engine, and stationary friction pads or brake shoes that are engageable with the drum or disc in applying a retarding force thereto by frictional contact. The degree of retarding action is dictated by the force with which this frictional contact is applied. A Prony brake as described may be used simply as a power absorption device or, in combination with torque measuring means, as a dynamometer for testing the under-load performance characteristics of the prime mover.
Once widely used as a dynamometer absorption unit, the Prony brake design has become less popular because of the general belief that its maintenance is too high, its life expectancy too short, and its load stability too variable. Still, the Prony brake configuration offers many advantages in the dynamometer field in terms of economy of design and load varying characteristics. The present invention represents improvements in the basic Prony brake design substantially enhancing its attractiveness with respect to the aforementioned practical applications.
A friction brake operated as a dry, nonlubricated device has theoretically more torque/horsepower capacity per unit cost than any absorption device available. My earlier invention disclosed and claimed in U.S. Pat. No. 3,068,689 involves a dry brake absorption unit which is submersed under water in order to provide additional power absorption capacity. The disclosure of U.S. Pat. No. 3,068,689 is hereby incorporated by reference in the present application. A dry brake application operates very much the way that friction material is intended to be used in that a certain amount of breakdown of both the friction block, or material, and the metal casting itself occurs. This breakdown process exposes fresh contact points of both members in maintaining a reasonably constant and effective coefficient of friction until either one or both members require either reconditioning or replacement. The operation of a "dry" brake, however, creates problems relating to the "sluff off" of friction material particles which accumulate as dust. This dust is very destructive to bearings and seals and its accumulation causes recirculation problems and a somewhat unstable loading characteristic.
In any application save perhaps that of an automobile brake this type of nonlubricated friction engaging mechanism would have a usable lifetime of short duration. The accumulated residue of metal upon metal during the wearing process limits the lifetime of a conventional automobile brake to perhaps less than 100 hours of continuous, high energy absorption, rotational engagement. A dynamometer designed on the same limited operational basis as an automobile brake would be a design failure. A dynamometer application requires thousands of hours of accumulated operation before the occurrence of a major failure.
Water is the most common means for absorbing the mechanical equivalent of the heat generated in the power absorption process. At the same time, water is the most common limiting factor in the dynamometer operating environment. The vast majority of installations of power testing equipment suffer from shortages in water availability or are provided with expensive water supply installations. The major reason for this problem is that only 7% to 9% of the cooling capacity of water is utilized in the typical water impeller dynamometer.
The water impeller type of dynamometer requires a fixed amount of water usage which cannot be improved due to the formation of steam pockets on the receding edge of the impeller vanes. This negative pressure on the nonpressurized side of the impeller vane causes water to boil at a reduced temperature resulting in turbulence and a severe limitation on the impeller's capacity to provide a continuous, constant load or create torque resistance. In addition, the metallic impeller is subject to erosion in the form of sublimation due to the formation of these steam pockets on the impeller vanes. Not only is the impeller type of dynamometer limited in its loading capacity, but it also is severely temperature limited in that mineral dropout, or sublimation, becomes a very serious problem at temperatures above 140.degree. F. The Prony brake type of dynamometer is thus a much more economical and efficient load absorption device than its impeller counterpart.
Another limitation in conventional dynamometer design involves the failure to take advantage of the full cooling capacity of water. At atmospheric pressure, water boils at 212.degree. F. This provides 72.degree. of heat usage per pound of water (also 72 BTU's) over that available in an impeller dynamometer. Water's big advantage, however, lies in its latent heat of evaporization which is 966 BTU'S per pound of water evaporated or an additional 966 BTU's plus the 72 BTU's in excess of the 140.degree. impeller dynamometer limitation. The resulting 1,038 BTU's is available if the internal pressure of the evaporative device can be maintained at atmospheric pressure. Also operating in these conditions avoids excessive temperatures and the unsafe conditions associated therewith.
Prior art dynamometers incorporating a lubrication system also suffer from various limitations. In these systems a certain amount of lubrication provides protection of the internal contacting members and continues to do so up to a point where the lubricant either breaks down into its organic components or volatizes ("cooks off"). The breaking down of the organic lubricant results in the formation of a glazed coating on the friction material and a reduction in the torque engaging capacity. A skipping action can occur with the introduction of the lubricant between two smooth surfaces such as a smooth (glazed) brake lining and a smooth metal brake drum. Without this glazing, or breakdown, the service life of the lubricated brake unit is virtually unlimited. In addition, current dynamometer lubrication systems are unable to provide uniform lubrication between brake drum units or along the entire length of a brake unit, fail to confine the lubricant in a closed environment which becomes an important factor when expensive lubricants are used at high temperatures, and apply the lubricant to the engaging surfaces in a nonflowing manner.
The present invention does not suffer from these limitations in that it offers a Prony brake type of load absorption unit, or dynamometer with conventional measuring instrumentation coupled thereto, which has a circulating, closed lubrication system which not only substantially extends the usable lifetime of the components therein, but also allows the unit to achieve larger loading capacities without the need for increasing the unit's heat dissipation capacity.